Characterization of a Xenopus laevis CXC chemokine receptor 4: implications for hematopoietic cell development in the vertebrate embryo

Previous reports have shown that the G(i)-protein-coupled CXC chemokine rec eptor 4 is activated by stromal cell-derived factor 1 (SDF-1). The receptor is present in many cell types and regulates a variety of cellular function s, including chemotaxis, adhesion, hematopoiesis, and organogenesis. To exa mine the role of CXCR4 as a regulator of organogenesis in the vertebrate em bryo, we have isolated a cDNA encoding the Xenopus laevis homologue of CXCR 4 (xCXCR4). The encoded polypeptide was functionally reconstituted with rec ombinant G(12) in baculovirus-infected insect cells. Although xCXCR4 shares only 3246 of its extracellular residues with mammalian CXCR4, it is indist inguishable from human CXCR4 in terms of its activation by human SDF-1 alph a and SDF-1 beta. The fact that only 19 of these residues are specifically present in the extracellular portions of CXCR4 suggests that these residues may be involved in recognizing SDF-1 and/or mediating CXCR4 activation by SDF-1. Xenopus CXCR4 mRNA expression was up-regulated during early neurula stages and remained high during early organogenesis. Whole mount in situ hy bridization analysis showed abundant expression of xCXCR4 mRNA in the nervo us system, including forebrain, hindbrain, and sensory organs, and in neura l crest cells. xCXCR4 mRNA was also detected in the dorsal lateral plate, t he first site of definitive hematopoiesis in the amphibian embryo correspon ding to aorta-gonad-mesonephros or pam-aortic splanchnopleura in mammals. T his observation suggests that SDF-1 and CXCR4 are involved in regulating th e migratory behavior of hematopoietic stem cells colonizing the larval or f etal liver. The hematopoietic defects observed in mice lacking SDF-1 or CXC R4 may, at least in part, be explained by a disturbance of this migration.